In a conventional forward converter, as shown in FIG. 1, a dc input voltage is connected to a solid state switch Q1 (101) through a transformer T1 primary winding 102. The resulting voltage developed across T1 secondary 103 is connected through diode D1 (104) and inductor L1 (105) to charge a capacitor C1 (106) and develop an output voltage. Current flowing into this secondary circuit is reflected back through transformer T1 so that switch 101 collector current is the load current, with variations due to fluctuations in inductor 105 current, and divided by transformer T1 primary to secondary ratio.
When switch 101 turns off inductor 105 inductor current carries diode 107 cathode negative to the negative output rail and the output voltage is maintained by capacitor 106 as inductor 105 current decays linearly with time. The converter maintains the desired output voltage by cycling switch 101 on and off at high frequency and controlling the ratio of turn on to turn of time. The current in inductor 105 ramps up linearly with time when switch 101 is turned on and ramps down when switch 101 is turned off and so displays a saw-tooth waveform for which the average current is the output current.
When transistor 101 turns off the collector voltage rises rapidly as the flux linking T1 primary 102 collapses. While T1 secondary 103 voltage is still positive the main moving force is inductor 105 secondary current reflected back to primary 102. This pulls the collector of transistor 101 positive until T1 primary 102 and secondary 103 voltages are substantially zero. The rate of increase of voltage at this time is rapid and occurs before switch 101 has had time to fully turn off. Thus turn off switching losses are high. At this point inductor 105 becomes clamped to the output negative rail and current flow back to primary 102 from this source is cut off. T1 primary 102 still has the magnetizing current, due to T1 primary 102 inductance, and this current carries switch 101 collector positive. If not clamped in some way the voltage tends to infinity and is destructive to switch 101, T1 and diode 104.
Conventional forward converters employ a variety of means to limit the collector swing of switch 101. Some resonant forward converters and the circuit described here, make use of a resonant primary circuit both to limit the rate of voltage increase when switch 101 turns off and also to control the maximum voltage reached by the collector of transistor 101.
A basic resonant forward converter is shown in FIG. 2. Capacitor C2 (201) is added and T10 primary inductance (Lp) controlled so that capacitor 201 and T10 primary inductance Lp form a parallel resonant circuit. It is assumed that there is a low impedance path to high frequency currents across the dc input. When switch 202 turns off inductor 203 current reflected through T10 flows into capacitor 201 and switch 202 collector voltage initially rises at a rate determined by capacitor 201 and the current flowing into it: inductor 203 current divided by T10 primary to secondary ratio. The rate of increase of voltage is relatively slow and switch 202 has time to fully turn off before an appreciable voltage has been developed across it. Thus turn off switching losses are very low. When T10 primary voltage becomes zero, and inductor 203 current flow is restricted to the secondary circuit, the remaining energy in T10 primary 204 due to magnetizing current now carries capacitor 201 and transistor 202 collector into a sinusoidal waveform for which the maximum voltage depends on circuit parameters. There are many variations of this basic resonant forward converter topology.